474 ■ CAMBKIAN BRACHIOPODA. 



of the valves, however, showed, together with some suggestions of the last-mentioned genus {Lingula), unmistakable 

 marks of the genus Obolus, so that the species, which would not be assigned to any of the other groups, had to be ranked 

 in a special subgenus of Eichwald's.genus. 



Type. — Obolus {Leptembolon) lingulxformis Mickwitz (PL XIV, figs. 5, 5a-b). 

 Lingulepis Hall [1863, p. 129] is a LinguleUa-]ike shell with the ventral beak much atten- 

 uated. Type: Lingula acuminata Conrad. 



For the species hitherto referred to Lingulella that are now referred to other genera or 

 subgenera see pages 58-62. 



Lingulella acutangula (Roemer). 

 Plate XVII, figures 1, la-o. 



Lingula acutangula Roemer, 1849, Texas, p. 420. (Described in Latin and discussed in German as a new species.) 



Lingula acutangula Roemer, 1852, Kreidebildungen von Texas, p. 90, PI. XI, figs. lOa-b. (Described in I/atin and 

 discussed in German.) 



Lingulepis acutangulus (Roemer), Sohitchekt, 1897, Bull. U. S. Geol. Survey No. 87, p. 259. (Merely changes generic 

 reference.) 



Obolus (Lingulella) acutangulus (Roemer), Walcott, 1898, Proc. U. S. Nat. Mus., vol. 21, pp. 392 and 394; PI. XXVII, 

 fig. 6; PI. XXVIII, figs. 1 and 2. (Mentioned in the text in discussion of Lingulella. Figs. 6, 1, and 2 are repro- 

 duced in this monograph, PL XVII, figs. Ic, If, and Ih, respectively.) 



General form elongate ovate with the ventral valve subacuminate and the dorsal valve 

 ovate in outline. When the ventral valve is slightly compressed the cardinal slopes are often 

 crushed down over the area about the pedicle groove, so as to make the outline of the valve more 

 acuminate. This appears to be the case with the type figure of Roemer, and it is partly shown 

 by Plate XVII, figure le. There is considerable range of variation in the outline of the valves 

 (PI. XVII) . The convexity of the two valves is fairly strong, and it is nearty the same in each, 

 except that the dorsal valve curves inward more rapidly toward the beak. A ventral valve 

 9 mm. in length has a convexity of 1.25 mm., and a dorsal valve 10 mm. long arches 1.5 mm. 

 above the plane of the margin. 



Surface of the shell marked by radiating and concentric strise and lines of growth. The 

 radiating strise are usually indistinct, but in a number of shells they are well marked or inter- 

 rupted somewhat by irregular rounded lines that alternate, blend, and disappear without any 

 apparent system (PI. XVII, fig. 1 1). The concentric striae are fine, usually having an unbroken 

 curvature, but in some instances they are broken into short vmdulations, such as characterize 

 the surface of several other species of Lingulella. In some examples the undulations are close 

 and pronounced, and form a fine broken reticulation that is discernible only with a strong mag- 

 nifj^ing glass. Wlien the outer layer of the shell is partly exfoliated the radiating strife appear 

 as sharp, clear-cut lines, but they do not extend to the inner surface of the shell (PL XVII, fig. 1 1) . 

 The interior surface is usually marked hj strong puncta? which vary greath" in number and 

 arrangement. This is illustrated by the casts of the interior of several shells, where the punct« 

 are represented by papillse. On figure Ic the papillfe are large and somewhat scattered, while 

 in figure la the}'' are smaller and arranged in concentric lines quite to the edge of the shell. 

 In the dorsal valve the punctse are not quite so strong and they do not extend out to the margin 

 (PL XVII, figs, li, Ij, and Ik) ; in specimens showing the interior scars and markings the punctse 

 are less numerous (figs. If, Ih, and Ij). 



The shell is thicker than m most species of the genus, in this respect resembling in a modified 

 degree the type of Obolus, 0. apollinis Eichwald. The shell is formed of a thin outer layer 

 and several thinner layers or lamellae that are slightly obhque to the outer layer (fig. Ini), a 

 structure quite similar to that of the shell of the recent Lingula anatina Bruguiere. 



The longest ventral valve m the collection has a length of 14 mm., and a maximum width 

 of 10 mm. Roemer's figure [1852, PL XI, fig. 10] is larger, 18 mm. by 13 mm. The average 

 length of the ventral valve is from about 8 to 10 mm., and 'of the dorsal valve 6.5 to 8.5 mm. 

 The relative dimensions of the two valves may be seen by a comparison of figures 1 and li, 

 Plate XVII. 



